Backlit display devices, such as liquid crystal display (LCD) devices, commonly use a slab or wedge-shaped lightguide. The lightguide distributes light from a substantially linear source, such as a cold cathode fluorescent lamp (CCFL), to a substantially planar output. The planar light output of the lightguide is used to illuminate the LCD.
As is well known, the purpose of the lightguide is to provide for the distribution of light from the light source over an area much larger than the light source, and more particularly, substantially over an entire output surface area of the lightguide. In slab, wedge and pseudo-wedge lightguides, light typically enters the lightguide along an edge surface and propagates between a back surface and the output surface from the edge surface toward an opposing end surface of the lightguide by total internal reflection (TIR). In slab and certain wedge lightguides, the back surface includes structures, e.g., dots in a pattern, facets, etc. A light ray encountering one of these structures is redirected, i.e., either diffusely or specularly reflected, in such a manner that it is caused to exit the output surface. In other wedge lightguides, light is extracted by frustration of the TIR. A ray confined within the lightguide by TIR increases its angle of incidence relative to the plane of the top and bottom wall, due to the wedge angle, with each TIR bounce. The light eventually refracts out of the output surface at a glancing angle thereto, because it is no longer contained by TIR.
More recently, light emitting diodes (LEDs) have been increasingly used as a light source for backlighting applications in LCDs, such as, for example, in conjunction with light guides or light pipes. FIG. 1 details a conventional backlighting device 10 currently in use within the industry. The system 10 includes a light source 12; a light source reflector 14 and a lightguide 16. The light source 12 may be a LED or bank of LEDs that provides light to an input edge surface 18 of the lightguide 16. The light source reflector 14 may be reflective film that wraps around the light source 12 forming a cavity 15. The lightguide 16 may be an optically transmissive monolithic wedge including a back surface 20 and an output surface 22. The lightguide 16 includes an end surface 24 opposing the input edge surface 18. Each of the back surface 20 and the output surface 22 is substantially planar with the back surface converging at the wedge angle toward the output surface. This configuration for the lightguide 16 provides for propagating light from the input edge surface 18 between the back surface 20 and the output surface 22 toward the end surface 24 by TIR and for the extraction of light by frustration of the TIR. An LCD (not shown), is placed over the output surface 22 of the light guide, thereby rendering LCD display legible.
The light guide 16 is optically coupled to the light source 14 and serves to channel the light along its entire length and is designed such that light is reflected up and out of the output surface 22. The color of the backlighting can be modified by changing the light source 12 to a different color, such as by changing the LED or by adding or changing a phosphor material associated with the LED. If a blue illumination is needed, a blue LED is used for the light source 12. Likewise, a red LED is used if red illumination is needed for the light source.
A type of LED increasingly utilized in backlighting is a white LED device. The white LED device, as the name implies, emits radiation that appears white to an observer. In one example, this is achieved by combining an LED, which emits a blue light, and a phosphor such as cerium activated yttrium aluminum garnet (Y3Al5O12:Ce3+). The blue LED emits a first radiation typically with peak wavelength of 450 to 500 nanometer (nm). The phosphor partially absorbs the blue radiation and reemits a second broadband radiation with peak wavelength of 560 to 580 nm. The combination, also referred to as a composite radiation, of the second yellow radiation together with the unabsorbed first radiation gives a white appearance to the observer. Although offering some advantages, conventional blue chip and yellow phosphor LED based backlighting systems offer limited control over correlated color temperature (CCT), color rendering index (CRI), and color gamut for the display.
In addition, while somewhat effective, conventional LED based backlighting devices place the phosphor onto or immediately adjacent the LED chip in a phosphor layer. Placing the phosphor next to the LED chip has been observed to reduce the conversion efficiency due to phosphor saturation effects, thermal quenching and other optical losses. This can lead to significant reduction in lumen output, reducing the advantages of such backlighting systems. In addition, saturation can lead to severe and undesirable changes in the color point of the device with variations in drive current.
Thus, there is a need for a new LED based backlighting system wherein saturation and other output loss effects are minimized while still maintaining control over CCT, CRI and color gamut for the device.